Solid fossil fuel, such as coal, is an important energy source, particularly for power generation. Pollutants emitted from coal combustion, however, are a major source of air pollution. Of the pollutants from coal combustion, nitrogen oxides (NOx) have attracted extensive attention.
There are two primary sources of NOx generated during combustion: fuel NOx and thermal NOx. Fuel NOx is NOx formed due to the conversion of chemically bound nitrogen (fuel nitrogen). Fuel nitrogen (or char-N) is released in several complex processes. The primary initial product of combustion is either HCN or NH3. HCN is either oxidized to NO or reduced to N2. If the gases are oxidant or the fuel is lean, NO will be the dominant product of fuel nitrogen. If it is fuel rich, HCN is reduced to N2 by CO or C (char) on the coal char surface.
Thermal NOx refers to NOx formed from high temperature oxidation of atmospheric nitrogen. Thermal NOx formation is an exponential function of temperature and a square root function of oxygen concentration. A lower combustion temperature or a lower oxygen concentration yields a lower NOx. Therefore, the production of thermal NOx can be controlled by controlling the reaction temperature or the oxygen concentration. However, a lower combustion temperature or a lower oxygen concentration leads to an inefficient burning of coal, i.e., a slow burning rate. A slow burning rate may result in an incomplete burning of coal and a prolonged burning of coal.
Various technologies have been developed to reduce NOx emission. These technologies either reduce the combustion temperature or manipulate the oxygen concentration. The first is called “dilution based combustion control technique,” and the latter is referred to as “stoichiometry based combustion control technique.” The dilution based combustion technique introduces inert gases such as water or flue gases to reduce the flame peak temperature. The stoichiometry based combustion technique involves lowering the oxygen concentration in the flame zone and generating a reducing atmosphere, thus allowing NOx to be reduced. Examples are low-NOx staged burners and OS combustion, e.g., over-fire-air and burner-out-of-service. These techniques control NOx generation by providing air staging to create an initial fuel-rich zone (partial combustion zone) followed by an air-rich zone to complete the combustion process. These low-NOx burners can reduce the NOx emission to 0.65 to 0.25 pounds per million BTUs. Another type of NOx control technology is gas reburning. The reburning technology can lower the NOx emission to 0.45 to 0.18 pounds per million BTUs.
However, these NOx reduction techniques are less than adequate. For example, they cannot meet the emission requirements (less than 0.15 pounds per million BTUs) under the U.S. Clean Air Act. Additionally, in almost all low-NOx combustion techniques, the combustion time has to be increased significantly. As a result, the boiler size must be increased to accommodate the long combustion time so that coal combustion can be completed at an economically acceptable level. Consequently, almost all the NOx control technologies require significant capital investment, and the cost of operation is high.
Recent studies have shown that feeding coal with high-temperature gas could significantly reduce NOx emission and unburned carbon in fly ash. In the combustion process with high-temperature gas, the fuel nitrogen is devolatilized rapidly, and reduced to nitrogen during devolatilization and combustion in a fuel rich zone.